The present invention relates to voltage controlled oscillators (VCOs). More specifically, the invention pertains to VCOs of the type used in frequency synthesizers for synthesizing ultra-high frequencies (UHF) and above.
The design of high performance VCOs of the type used in portable radio frequency synthesizers requires the optimization of several parameters. The most significant such parameters are the ratio of output power to phase noise, and current drain. It is generally accepted that junction field effect transistors (JFETs) are the easiest devices to optimize. Therefore, they are generally used in low current high performance VCOs. However, they are only useful for supplying output power output in the low milli-watt range. In addition, JFETs suffer a significant loss in gain with rising temperature.
Bipolar transistor devices can be used to provide higher power levels. However, they provide adequate performance only under certain conditions. When VCOs must be configured to operate at UHF and above, the design task becomes especially difficult for the following reasons. Passive component Q's are lower at UHF than they would be at lower frequencies. Device and interconnect parasitics limit the degree to which phase noise and closed loop gains may be optimized. Phase noise performance degrades as frequency increases. This degradation is approximately 6 dB/octave.
Thus, it is difficult if not impossible to build a highly stable, highly reliable VCO for operation in the UHF range. The problem is made even more difficult when switching circuitry must be incorporated into the radio to move the VCO in frequency to provide a transmit offset.
The literature shows several circuit arrangements which incorporate both transistors and FETs. The following list is intended merely as a representative sample of the patent literature.
U.S. Pat. No. 4,216,443 - Zaderej (Aug. 5, 1980), and PA1 "The Goral Oscillator" published in Radio Communication, Vol. 52 No. 5 pp. 359-360, May, 1976 by the Radio Society of Great Britain.
As will be appreciated from the following discussion of the invention, neither of the arrangements shown in these reference documents suggest the central concept of this invention.